Why do bubbles in Guinness go down?
With St. Patrick’s Day fast approaching, it is the best time to recall some interesting physics associated with Ireland’s favourite drink, Guinness.
Anyone who has ever tried Guinness knows that bubbles in it sink. In fact, in one’s everyday life, one rarely comes across a more counter-intuitive phenomenon, challenging equally the imagination of a university professor, as well as that of Bill, John, and Harry from the local pub.
It took putting three pieces of the puzzle in their places to unravel the mystery. Firstly, Andy Alexander and Dick Zare showed that this effect is real and not an optical illusion and, secondly, Robert Service carried out computer simulations which demonstrated that the bubbles are driven by a downward flow, the velocity of which exceeds the upward velocity of the bubble due to the Archimedean force. The existence of such a flow near the wall of the glass cannot exist without an upward flow somewhere in the interior – but the mechanism of this circulation remained unclear.
The third and final piece was added to the puzzle in our paper with Cathal Cummins, where we showed that the observed circulation is driven by the shape of the glass. If it narrows downwards (as the traditional Guinness glass, the pint, does), the flow is directed downwards near the wall and upwards in the interior, so that sinking bubbles will be observed. If the glass widens downwards, the flow is opposite to the one described above and only rising bubbles will be seen.
To understand the reason of the asymmetry, assume that the bubbles are initially distributed in the glass uniformly. When they start moving, however, their upward motion immediately creates a bubble-free layer near the sloping wall. The density of the pure liquid in this layer exceeds that of the bubbles/liquid mixture elsewhere, which makes the near-wall layer slide downwards – and, thus, gives rise to the observed circulation. One can similarly argue that, should the glass widen downwards, the rising bubbles accumulate near the wall, and the smaller density of the near-wall layer would drive it upwards.
There is a simple experiment illustrating all of our arguments: if Guinness is poured into a tall cylindrical container – a laboratory measuring cylinder, for example – and the container is tilted, bubbles will be observed to move upward near its upper surface and downward near its lower surface (see this video), in precise agreement with the proposed mechanism.
One important application of this research might be in designing Guinness glasses in which settling occurs faster or, perhaps, glasses where regions of high/low density of bubbles form pretty patterns. Our results, however, also illustrate a more important principle: simulating a phenomenon does not necessarily equal understanding it. The sinking bubbles in Guinness are a reminder of the importance of the synergy between complex simulations including all possible effects and simpler mechanistic modelling clarifying the essence of a phenomena.
Professor Eugene Benilov enjoys the occasional pint of Guinness and is a PI on the MACSI grant.
William Lee leads the Industrial Mathematics group at the University of Portsmouth. His interest in Guinness is purely scientific.
The research described in this article was funded by Science Foundation Ireland.